Layered clothing is a term describing a way of dressing using many garments that are worn on top of each other. Some of the layers have different, largely non-overlapping, functions. Using more or fewer layers, or replacing one layer but not others, allows for flexible clothing to match the needs of each situation. Two thin layers can be warmer yet lighter than one thick layer, because the air trapped between layers serves as thermal insulation. Layered clothing is particularly relevant in cold climates, where clothing must at the same time transfer moisture, provide warmth, and protect from wind and rain. In a hot and dry climate, clothes have very different functional requirements: they must block the radiation from the sun, and allow for sufficient air circulation. Therefore, layered clothing in the sense used in this article is largely irrelevant in hot and dry climates. The wicking layer moves the sweat from your skin, where it can be absorbed by the other layers.
The purpose of the inner layer is to draw the sweat away from the skin to the next layers, which makes the wearer feel warmer and more comfortable. The transfer of moisture happens due to capillary action, sometimes called wicking. The used materials are called wicking materials. When moisture has moved from the skin into (nonabsorbent) clothing, it has more surface area and will evaporate faster. If a piece of clothing does not transfer moisture well, it is not strictly an inner layer garment at all, but simply a comfortable mid-layer garment.
Synthetic materials such as polyester, polyethylene, and microfiber-based fabrics are inexpensive. They have excellent water wicking properties. They can also carry specialist finishes, such as anti-bacterial agents which reduce odors, and insect repellent. However, in the absence of such anti-odour treatment they quickly become notoriously foul-smelling. This is because their hydrophobicity causes them to strongly absorb the short-chain fatty acids that are responsible for body odor.
The mid layer is needed in cold weather to provide additional insulation. The use of multiple thin layers facilitates adjustment of warmth. The mid layer should be more loose-fitting than the inner layer, as this leaves insulating air between the layers. However, if best possible moisture transfer is desired, too great a gap between any adjacent layers of clothing may reduce the moisture transfer by capillary action from one piece of clothing to another. On the other hand, very loose-fitting layers can allow more removal of moisture (and heat) via air circulation.
Synthetic Fiberfill such as polyester fiber is used similarly to down, but does not have as good a warmth:weight ratio. However, it is less expensive, provides better insulation when wet, dries quickly, and absorbs very little moisture. There are brands of very fine fiberfill like Thinsulate, PrimaLoft or Thermolite, that provides higher warmth for a given thickness.
The outermost clothes are called the shell layer, but only if they block wind or water, or have good mechanical strength. Ideally the shell layer lets moisture through to the outside (that is, is breathable), while not letting wind and water pass through from the outside to the inside. While this is enabled to some degree by modern materials, even the best and most expensive materials involve a slight trade-off between breathability and water- and wind resistance.
Water resistant (soft shell) materials generally block water only partially, however as technology in the outdoor industry moves forward more fully waterproof soft shells are emerging such as polartec neoshell or DryQ Elite. On the other hand, they are usually more breathable and comfortable, thinner, and cheaper than completely waterproof materials. Water-repellent coatings are often used. Before waterproof-breathable shells were invented, the "60/40" (60% cotton, 40% nylon) parka was widely used. Soft shells are not water "proof".
Combining different garments in layers can be used to create a variety of outfits. This provides similar practical benefits for practical layering, in that the wearer can shed layers according to changes in temperature, and is also a way of making use of clothing to produce different looks and mix colors in various ways. It saves money for the consumer, who can create a totally new look simply by swapping out one piece of the outfit.